Acetic acid and acetic anhydride are used for a variety of applications including the production of vinyl acetate, ester production, the formation of vinegar, and as a solvent. Acetic acid is generally produced by the carbonylation of methanol and/or methyl acetate in the presence of a catalyst containing both rhodium and iodide.
However, even after distillation, the acetic acid or acetic anhydride contains a small amount of iodide impurities. This is problematic particularly because even a small amount of iodide will reduce the lifetime of catalysts in downstream processes. For example, the majority of acetic acid produced is used in the production of vinyl acetate and cellulose acetate by processes using sensitive, expensive catalysts containing metals such as gold and palladium. Since halides, especially iodide compounds deactivate or “poison” gold and palladium catalysts, starting materials that are essentially halide-free are preferred.
Even after extensive purification, carbonylation reaction products typically contain contaminants from the original halide compounds added to the reaction medium as a promoter prior to the carbonylation reaction and new halide compounds generated during the carbonylation reaction. While the exact composition of these impurities is unknown, the impurities comprise a mixture of hydrogen iodide, alkyl iodides such as methyl iodide and hexyl iodide, and iodide salts. Other iodide compounds, including aromatic iodides, may also be present in the mixture.
The industry standard for halide contaminants in acetic acid is 10 parts per billion (ppb) or less. However, poisoning effects on precious metal catalyst are generally cumulative and irreversible as in the case of iodide contamination of catalysts for vinyl acetate production. Consequently, the removal of as much iodide contamination as reasonably possible is desired. Further, since all halide contaminants (e.g., HI, CH3I, C6H13I, and aromatic iodides) will poison downstream catalysts, all iodide contaminants, not just the short chain aliphatic iodides, should be removed.
There are a number of resins used in acetic acid iodide purification. Most of these resins contain iodine-reactive metals such as silver, mercury, copper, lead, thallium, palladium or combinations of these metals known to react with iodine-containing compounds to form insoluble complexes. For example, in U.S. Pat. No. 4,615,806, the removal of these impurities is achieved with a macroreticulated strong acid cation-exchange resin which is stable in the organic medium and has at least one percent of its active sites converted to the silver or mercury form, presumably by cation-exchange.
Other resins, including macroreticulated silver-exchanged resins are described in U.S. Pat. Nos. 5,139,981; 5,220,058; 5,227,524; 5,300,685; 5,416,237; RE35615; 6,017,969; 6,197,997; 6,657,078. WO 2008/03446 and JP 09 291058 describe silver-exchange resins having specific particle and pore size. These resins and methods are each directed to the removal of iodide compounds. However, there is need for resins having a greater effectiveness and/or a longer life for use in removing iodide contaminants from solvents or solutions.